1. Field of the Invention
The invention relates to a method and apparatus for an image capture instrument, and more particularly to a method and apparatus of look-up table (LUT) of a scanner instrument, that enables the reduction of memory size of the look-up table.
2. Description of the Prior Art
Optical scanners are used to capture and digitize images. For example, an optical scanner can be used to capture the image of printed matter on a sheet of paper. The digitized image can then be electronically stored and/or processed with character recognition software to produce ASCII text. The typical optical scanner includes, such as a light source, a linear array of photoelectric sensing elements (generally a CCD sensor), an analog amplifier, an analog-to-digital converter (ADC), a controller and a random access memory (RAM).
As depicted in FIG. 1, each photoelectric sensing element 110 of an imaging system, such as a scanner, converts the light it receives into an electric charge. The magnitude of the charge depends on the intensity of the light and the exposure time. Next, the charges from each of the photoelectric sensing element 110 are converted into analog voltages via the analog amplifier (not shown). The analog voltages are digitized (internal M-bit digital signals) by the analog-to-digital converter 120 for digital image processing and storage in the RAM. One problem with a current scanner is that the internal M-bit digital signals produced by the light-sensing elements are not directly proportional to the intensity of light reflected from the image. In other words, the output of the light-sensing element is non-linear when related to the light input for the full available dynamic range of the sensing element. There is always a sensing element translation between light input and digital internal signal conversion for printer processing. A gamma look-up table 140 that follows a task of shading compensation 130 can implement the translation of the M-bit internal digital signals. Then the output from gamma look-up table 140 is translated into a host through a memory 150.
The look-up table stores predetermined mapping information, that follows a curve (usually, a monotone curve is chosen) and acts as a memory to provide an output each time it receives an address. However, while an input signal has more bits than ever this creates a disadvantage for the utility of the look-up table as the size increases dramatically. Shown in FIG. 2, an input signal Si follows the format of 16-bits per pixel and 16-bits per pixel for an output signal S0. It is necessary for the gamma look-up table 140 to consume up to 370K bytes to map the input signal S1 into an identity mapping terms for getting the better output signal S0. In order to reduce the cost of the look-up table, interpolation is applied on the mapping of the look-up table. Double period-reading or value-reading are necessary for interpolation, that is disadvantageous to performance of high-speed design. Alternative resolution is to execute the task of mapping in the host. However, the method increases time and memory consumption.